What is an NRC?
NRC stands for Noise Reduction Coefficient. The method by which NRC is ultimately obtained can be: the Reverberation Room Method (ASTM C423) or the Impedance Tube Method (ASTM C384). The Reverberation Room Method is the more popular of the two in terms of tests conducted on acoustical room treatments. How this method works: Approximately 72 square feet (or more, but not less) of material is placed on the floor of a Reverberation Chamber – a big room with (usually) all hard, concrete surfaces (the opposite of an ”Anechoic” Chamber – a room with no echoes) – and the change in absorption from the empty room to the room with the treatment area on the floor is measured. A kind of ”Before and After” test.
The final result of the calculations is reported as Sabin absorption coefficients (”Sabin alphas” or ”aSAB”) in octave bands from 125 Hz to 4000 Hz. For a convenient, single-number rating, the Sabin alphas for 250, 500, 1000 and 2000 Hz are averaged and the result is the Noise Reduction Coefficient, or NRC. The alphas in the individual octave bands are interpreted as the relative sound absorbed over the octave band range. The higher the number, the more sound is absorbed.
Those of you who are math savvy will notice right away that an NRC doesn’t tell you much. Take a look at the following two columns of alphas with the same NRC:
*In accordance with standards, NRCs are rounded off to the nearest 0.05.
These two materials have identical NRCs, but do not perform identically in individual bands. If you want to get an idea of how an acoustical material actually performs, look at the alphas in the individual bands. If you want to get ”in the ballpark,” then you may find the NRC useful. NRCs are handy when comparing materials side-by-side, but only to a point. For example, drywall has an NRC of about 0.20 and 2” Studiofoam NRC is 0.80. Obviously, the Studiofoam is better at absorbing sound. On the other hand, 2” Studiofoam and 1”, 6# fiberglass both have NRCs of 0.80, but the alphas in different bands are not the same.
Some other useful absorption coefficient and NRC information:
• The ASTM C423 standard makes it possible to arrive at absorption coefficients and, therefore, NRCs that are greater than 1.00. This may be counter-intuitive since many references define the Sabin alpha and NRC as the ”percentage of sound absorbed” by a material. This treatment of alphas and NRCs as percentages, however, is not really accurate. The formulae used in the standard to measure absorption are dependent on: the decay of sound in the test room, room volume, room temperature and the area of Chamber floor covered by the test material. ”Sabin absorption” and ”Sabine alphas” come from the fact that the absorption is calculated using the Sabine equation. Nowhere in the ASTM C423 standard is there a reference to Sabine alphas being equal to percentages of any kind. Therefore, numbers greater than 1.00 are possible. This means that Sabine alphas are simply a representation of the relative amount of sound absorbed by the material. (Relative to the absorption without treatment in the room.) Higher numbers mean more absorption in that frequency band.
[For more on this, please see The Sabins at Riverbank by John Kopec. Also, the absorption coefficient, or alpha, of a material is sometimes calculated using the difference between incident and reflected sound intensity divided by the incident sound intensity. In other words, a percentage. Care should be taken not to equate this alpha with the Sabin alpha described above (We have made this mistake ourselves in the past!), as they illustrate two different properties. This may be the source of some of the confusion about absorption coefficients.]
- Alphas measured in the lab are going to be different from those measured in a ”real” room. However, using the alphas to predict the acoustics of a (usually large) room will get fairly good results. Predicted effects at low frequencies are usually the most ”different” from real world measurements because, in general, the uncertainty of the lab measurements increases as frequency decreases.
- Alphas calculated using Sabine’s method are only completely valid to predict the acoustics in large, reverberant spaces using the Sabin formula. Other formulae are available for different initial conditions (i.e., how the room starts out – volume, surface types, etc.), but using Sabine alphas in these equations is not purely ”correct.”
- Small rooms cannot be considered ”reverberant” in the true sense of the term – even with all hard surfaces. There is simply not enough volume in the room to consider this so. Instead, using absorbers for the treatment of early reflections, flutter echoes and ”room ring” is more appropriate. Placement is usually by experience and using techniques such as the mirror trick (view the Video Library for placement tips and tricks) referenced earlier. Note: Auralex does not use the Sabin equation to predict small room acoustics.
- The method of mounting used for the test specimen in the reverberation chamber can affect the numbers. Most materials for treatment of walls or ceiling are tested using what is called an ”A” mounting. Type ”A” mounting means the test specimen was laid directly on the chamber floor. Ceiling tiles are often tested using an ”E400” mounting. The ”E” designates a sealed air space behind the specimen and the number after the ”E” is the depth of the airspace in millimeters. Other mounting methods are available (”B”, ”D”, etc.), but are rarely used. (See ASTM E795 for more information.)
- The best way to use NRCs and alphas provided by Auralex and other companies is to compare performance of products. Be careful, though. Oftentimes, NRCs are used as marketing tools. Be wary of companies that offer absorption coefficients and NRCs without references to standards and mounting methods. All Auralex absorption products, unless otherwise indicated, are measured in accordance with ASTM C423 using Type ”A” mounting (see above). Direct comparisons to competitors and other materials can only be made if their testing methods are the same.
- The thickness (normal and angular) of an absorber does limit its low frequency performance. However, the low frequency performance of panel absorbers might be better than other audio ”experts” have led you to believe. Often, the limitations of low frequency performance is described as a function of the thickness of the material versus the ¼-wavelength of the lowest frequency that is affected by the material. In other words, if a material is 4” thick, that corresponds to a low frequency cut-off of:
f = low frequency cutoff (Hz)
c = speed of sound in air (usually about 1130 ft/sec)
t = thickness of absorber (ft)
This is misleading. This would be valid if the panel was only placed at normal incidence to a sound source. ”Normal incidence” means at 0°. A loudspeaker facing parallel to a wall would be considered normal incidence. When was the last time you saw this in a control room? Usually, walls, and hence absorbers, are placed at different angles to the sound source. The angular incidence of the sound on the panel increases the absorbers effective depth. Therefore, lower cutoff frequencies are achievable. To use the earlier example, this is the direct cause of absorption values for 4” materials below 850 Hz – the supposed low frequency cutoff for normal incidence. The bottom line: Absorbers such as 4” Studiofoam, Sunbursts, MAX-Wall, LENRDs, Venus Bass Traps, etc. can all absorb low frequency energy. Feel free to contact us to find out which one is right for you!
To purchase ASTM standards (or simply browse abstracts of the standards), please visit www.astm.org